1. Field of the Invention
The present invention relates to a process for preparing aliphatic polyester which is a biodegradable polycondensate and is useful as a medical material and a substitute of general purpose resin by direct polycondensation of a mixture of polyhydric alcohols with polybasic acids or a blend of said mixture with hydroxycarboxylic acids, and to the aliphatic polyester prepared by said process.
Aliphatic polyester can be hydrolyzed with relative ease in the presence of water and also decomposed by microorganisms.
Aliphatic polyester can also be hydrolyzed and absorbed in a living body and thus has been focused attention as a polycondensate which can be applied to the above uses.
2. Related Art of the Invention
As widely known, processes for preparing high molecular weight polyester from polyhydric alcohols and polybasic acids are based upon a deglycolation reaction of low molecular weight polyester having terminal hydroxyl groups.
Consequently, concentration of the terminal group is drastically lowered with increase in the molecular weight. A decomposition reaction due to the concentration of the ester at the ester exchange step is also added and thus restriction is found on the molecular weight increase. The tendency is particularly dominant in aliphatic polyester. For example, it has been found that the molecular weight attains a maximum value and successively starts to decrease in the preparation of high molecular weight saturated polyester by a conventional deglycolation reaction under reduced pressure.
In such cases, it is difficult so far as depending upon the conventional deglycolation reaction to prepare aliphatic polyester having a molecular weight which is satisfactory to form a tough film. In other words, the molecular weight obtained in the aliphatic polyester was low so that it was unable to form a film having characteristics for practical use.
Japanese Laid-Open Patent Publication HEI 4-180822 and 4-189823 describe preparation processes of high molecular weight polyester comprising adding, in the molten state above the melting point, an isocyanate compound having from 0.1 to 2 equivalents of an isocyanate group for an existing hydroxyl group to saturated polyester which has a number average molecular weight of 5,000 or more, a terminal group substantially consisting of the hydroxyl group and an acid component composed of a compound having 5 or more carbon atoms or a mixture of the compound.
Japanese Laid-Open Patent HEI 5-310898 has described a preparation process of high molecular weight aliphatic polyester comprising esterifying a glycol component with an aliphatic or alicyclic dicarboxylic acid component or its anhydride component and subjecting the resultant polyesterdiol to a deglycolation reaction at 180.degree..about.230.degree. C. in the presence of a catalyst under high vacuum of 0.005.about.0.1 mmHg.
U.S. Pat. No. 3,932,319 has also described a novel composition prepared by blending a naturally biodecomposed product with aliphatic polyester which has a reduced viscosity of 0.1.about.10 and is obtained by polymerizing at 250.degree. C. or above under vacuum of 1 mmHg or less and successively subjecting to a transesterification reaction under ultrahigh vacuum of 10.sup.-3 mmHg or less.
However, in the preparation process of aliphatic polyester at such a high reaction temperature as above, formation of impurities and discoloration of polymers due to the impurities lead to inevitable problems. Consequently, polymers obtained by using an isocyanate compound or conducting the reaction at a high temperature or under high vacuum are difficult to use for food packaging or medical applications.
Under such circumstances, stabilizers must be added to the polymers, Japanese Laid-Open Patent HEI 6-41288 has described a process for enhancing heat stability and reducing discoloration by reacting glycol with dicarboxylic acid or its anhydride to prepare an aliphatic polyester having a number average molecular weight of 5,000 or more and/or a weight average molecular weight of 30,000 or more in the presence of a metal compound catalyst in an amount of 0.001.about.0.5 part by weight for 100 parts by weight of formed aliphatic polyester, adding at the end of the reaction 0.01.about.3 parts by weight of a phosphorus compound for 100 parts by weight of said aliphatic polyester, and successively reacting said aliphatic polyester with 0.1.about.5 parts by weight of a polyisocyanate compound for 100 parts by weight of said aliphatic polyester to prepare a high molecular weight polyester having a number average molecular weight of 10,000 or more and/or a weight average molecular weight of 50,000 or more.
However, an isocyanate group existing in a high molecular chain as above tends to form a very toxic amine compound in the course of degradation and to accumulate the generated compound in soil.
As to hydroxycarboxylic acid, polyhydroxycarboxylic acid has been prepared by ring-opening polymerization of a cyclic dimer. For example, polylactic acid and polyglycolic acid have been individually prepared by ring-opening polymerization of lactide and glycolide which are cyclic dimers of lactic acid and glycolic acid, respectively.
However, in order to prepare lactide or glycolide which can be suitably used for the raw material of polymer, complex procedures including distillation under reduced pressure and repeated recrystallization are required. These complex procedures for purification require great labor and are unfavorable in economy. Additionally, the ring-opening polymerization cannot be carried out in the copolycondensation of hydroxycarboxylic acid which does not form cyclic lactones such as lactide and glycolide.
On the other hand, the preparation process of a high molecular weight polycondensate by direct polycondensation has not yet been established in a two component system composed of polyhydric alcohol and polybasic acid or a three component system obtained by further addition of hydroxycarboxylic acid.
The esterification reaction of the two component system or the three component system is a successive reaction and thus molecular weight of the product increases with extended reaction time. In the course of reaction, generated water or the monomer has an action to reduce molecular weight of the polycondensate by hydrolysis.
Consequently, it has been required for providing high molecular weight polyester to remove the generated water or the excessive monomer from the reaction system.